How does a P0171 code trigger the check engine light? This code will trigger the check engine light as follows:

The adaptive fuel strategy in the vehicle’s computer constantly monitors the fuel delivery system to make sure the engine is running at an optimum air to fuel ratio, which is 14.7:1. The computer adjusts injector pulse width to regulate the amount of fuel going into the engine. The oxygen sensors relay information to the Powertrain Control Module (computer), informing it of the oxygen content in the exhaust. This information is translated by the computer, and used to determine if more or less fuel is needed. The computer will then adjust fuel flow (and possibly other related engine operating characteristics), to keep the correct air fuel mixture. This loop continues as long as the engine is running. Please refer to our article Oxygen Sensor Code Diagnosis for more info on this system. A P0171 check engine light code is set when the computer has reached a rich calibration limit and can not add enough fuel to maintain the correct mixture.

Possible Causes:

Refer to our Oxygen Sensor Code Diagnosis article for a list of all possible causes. The following is an overview of P0171 check engine light code possible causes.

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